Soft Start, was Re: [Amps] Zero Crossing circuit

[Tomm Aldridge]

Soft starting with a phase angle controlled TRIAC works great for 
capacitive input filter PS inrush limit. Designs for single and 3 phase 
abound on the internet.  My biggest of this type was actually done on a 
transformerless boost converter operating from 3 phase.  Used 3 diode - 
SCR blocks and applied 100kHz drive (called picket fence) through 
trigger transformers.  The drive was generated by a slow ramp modulated 
phase angle controller timed for each phase.  Current sense would 
prevent the charging of a shorted capacitor.

Less complex but as effective is a series resistor that is switched out 
of the line with a voltage sensing relay.  The capacitor loads the 
output while charging so the voltage on the primary is less than 
necessary to fire the relay until the cap is pretty well charged.  At 
90VRMS or so, teh voltage sensing relay fires and shorts out the series 
resistor applying full line to the  primary of the xfmr.

The most "elegant", but also most massive is a saturable reactor with 
the control winding and a series R acting as the voltage sense.  This 
with a hard switched voltage sense relay to shunt it out of the circuit, 
makes a great and bullet proof current limiter.  A 3 phase 5kW unit 
weighs in at 200 lbs or so but is a joy to observe and listen to.

jeff millar wrote:
> Ed...
> 
> I've used zero voltage switched solid state relays twice in 2KW 
> amplifiers, one at 432 and one at 144 MHz.  The 432 amp included the 
> normal step start circuit, but once I noticed that the zero voltage 
> switching feature, I tried it without a step start in the 144 amp.  The 
> circuit switched on with very little drama, the lights blinked about the 
> same amount as when transmitting at full power.
> 
> One design note on high voltage power supplies suggested that extremely 
> high surge currents can damage electrolytic capacitors.  These surge 
> current maximizes when the turn on relay or switch happens to make 
> contact at the peak of the line cycle.  The zero voltage switching 
> function reduces the surge currents to approximately the same amount as 
> what it takes to recharge the caps at the peak of the line cycle.
> These amps showed no signs of RF getting into the switches, including 
> during operation at a big hilltop VHF/UHF multi-multi contest  group 
> with 1.5 KW on all the bands.  But for HF amps with a lot more RF 
> currents flowing around the shack, it's probably best to run the control 
> leads through an EMI filter similar to what you find on a AC power inlet.
> 
> One caution, the off impedance of these switches doesn't look that good 
> in the spec sheet, leading to the conclusion that an "off" power supply 
> could have 10 to 100's of volts on the output.  I didn't see this, the 
> voltage didn't rise above a volt or so, but maybe unit to unit variation 
> or operation at high temperature could cause more leakage.
> 
> hope this helps,
> 
> jeff, wa1hco
> 
> Ed Stallman <n5blz1 at houston.rr.com> writes:
> 
>>> Wondering if anyone has used a zero crossing circuit on a high voltage
>>> supply to reduce inrush current at startup? would this work for soft 
>>> start
>>> or would it just assist?
>>> Ed N5BLZ
>>>
>>>
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